I’ve been texting with a Famous Catholic Author™ about today, a Friday in the Christmas Octave, and the obligation to do penance. I’ve received email about it, too.
The Octave of Christmas does not have the “weight” of the Octave of Easter. Easter Friday outweighs the penance thing, but Christmas Friday does not.
Note can. 1251 in the 1983 Code.
Can. 1251 Abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays, unless a solemnity should fall on a Friday. Abstinence and fasting are to be observed on Ash Wednesday and Good Friday.
Friday in the Octave of Christmas is not a liturgical solemnity. Hence, we are obliged to do penance today, Friday in the Octave.
However, you can ask your parish priest to dispense you or commute your act of penance.
Can. 1245 Without prejudice to the right of diocesan bishops mentioned in can. 87, for a just cause and according to the prescripts of the diocesan bishop, a pastor [parish priest] can grant in individual cases a dispensation from the obligation of observing a feast day or a day of penance or can grant a commutation of the obligation into other pious works. A superior of a religious institute or society of apostolic life, if they are clerical and of pontifical right, can also do this in regard to his own subjects and others living in the house day and night.
Members of religious communities and third orders should consult their own regulations and review to whom they turn for dispensations.
You can substitute another form of penance for abstaining from meat. Make it penitential, however. Abstinence from meat has good reasoning behind it. For some, however, there abstinence from other things can be of greater spiritual effect.
Also, it may be that some local places have exceptions. For example, if you belong to St. Thomas Becket parish, then today is your patronal feast. Also, perhaps your conference of bishops made another ruling. I believe that is the case for England and Wales.